The present invention relates to a video switcher apparatus proper for a video display apparatus of displaying wide screen EDTV signals on a screen.
A screen configuration ratio (aspect ratio) of a current type television signal is 4:3 as shown in FIG. 9A. The Extended Definition TV (EDTV) system is in the spotlight recently as a television system which is able to reproduce a detailed image or a high definition image. A non-interlace scanning method having 525 horizontal scanning lines is applied to the EDTV system.
A wide screen type EDTV will next enter the spotlight as a next age of the EDTV system. This type employs non-interlace scanning having 525 horizontal raster lines similar to the current EDTV system. The screen aspect ratio will be 16:9 in a manner similar to a high definition television (HDTV) system.
It is difficult to accomplish a compatibility between a current type television signal and the wide screen type EDTV signal because their aspect ratios are different from one another. However, a so-called letter box system is proposed for the wide screen type EDTV signal as one method to accomplish compatibility.
According to the letter box system, if an image of 16:9 aspect ratio is reproduced on a screen of 4:3 aspect ratio, upper and lower screen areas become non-image areas as shown in FIG. 9C. In this drawing, slant line areas represent the non-image areas.
It is assumed that a main screen SM represents a common screen area to both the screens having different aspect ratios and a main screen signal (represented by SM) is displayed on the main screen area SM. In this instance, it is desired for the wide screen type EDTV signal that a relatively low frequency component of the wide screen type EDTV signal be assigned to the main screen signal SM and horizontal and vertical high frequency components (for the high resolution display) be assigned to a compensation screen signal SC. Then, the compensation screen signal forms non-image areas 1 and 2 as shown in FIG. 9C.
The compensation screen signal is used only for the EDTV receiver. The high definition image is reproduced by both the compensation screen signal and the main screen signal. The main screen signal can be used as the television signal by a conventional receiver. The upper and lower non-image areas 1 and 2 are accomplished by applying the wide screen type EDTV signal to the conventional receiver. If the compensation screen signal is inserted into the upper and lower non-image areas 1 and 2, the wide screen type EDTV video signal would be reproduced by the conventional receiver. It should be noted that the compensation screen signal is unnecessary for the conventional receiver because it is not used to produce an image. Thus, compatibility can be established between the wide type EDTV signal and the current TV signal.
Thus, the wide screen type EDTV signal may be converted into a format such that the main screen signal SM is positioned at the center portion of the raster and the compensation screen signal SC is positioned at the upper and lower non-image areas. Then, the converted signal may be transmitted.
The compensation screen signal includes high frequency components or the like along the horizontal and vertical directions, and these components act as a high frequency compensation signal. In this instance, for example, the horizontal compensation screen signal SCH is inserted into the upper non-image area 1 and the vertical compensation screen signal SCV is inserted into the lower non-image area 2.
Each of the compensation signals may be separated and inserted into the upper and lower non-image areas. As shown in FIG. 10, each of the non-image areas 1 and 2 may be separated by two horizontally. Moreover, the horizontal compensation screen signal SCH and the vertical compensation screen signal SCV may be separated and inserted into the non-image areas.
When the compensation screen signal is inserted, it is compressed horizontally or vertically and then inserted. Since a horizontal line number of the non-image areas to which the compensation screen signal is inserted is about one third of the horizontal line number of the main screen area, one horizontal line of the compensation screen signal corresponds to three horizontal lines of the main image signal. If the compensation screen signal is not compressed, the more non-image area is required.
The transmitted wide screen type EDTV signal can be displayed on the conventional receiver by processing the signal as discussed hereinbefore. On the other hand, the wide CRT type special receiver can display the wide screen type EDTV signal in the high definition mode by using the compensation screen signal.
If many video cameras outputting the wide screen type EDTV signals are used in a broadcasting station, it is necessary to switch the outputs from the many video cameras. The output switching operation is done for switching the video cameras as well as accomplishing special effects, such as a wipe operation.
Examples of the special effects are shown in FIGS. 11a, 11, 11b and 11c. A horizontal wipe image (FIG. 11b) or a vertical wipe image (FIG. 11c) is obtained by using an image of a camera A (FIG. 11a) and an image of a camera B (FIG. 11a').
A video switcher apparatus shown in FIG. 12 may be proposed to be used in this instance. This drawing is a block diagram of a two-channel video switcher apparatus for switching two cameras wherein, for example, a terminal 11 receives the wide screen EDTV signal "a" of FIG. 11a and a terminal 12 receives the wide screen EDTV signal "a". Reference numbers 13 and 14 represent gate circuits each of which executes a complement gate process in response to a wipe signal (key signal) applied to a terminal 15. A mixer 6 mixes the wiped wide screen EDTV signals with each other and applies the wiped EDTV signal "b" or "c" shown in FIG. 11b or 11c to a terminal 17.
In order to make the switching operation proper, the input signals "a" and "a'" should match with the wipe signal in spacial position.
As discussed hereinbefore, the wide screen EDTV signal is formed by inserting the compensation screen signal SC and the main screen signal SM into the raster in which the spacial position relationship therebetween is modified. Since the main screen signal SM and the compensation screen signal SC are wiped simultaneously, the screen is divided into, for example, the left half screen and the right half screen when the horizontal wipe is accomplished.
There is no special problem in the main screen. However, the high frequency components in the main screen signal SM of the left half L of the screen shown in FIG. 11b cannot be compensated only by the compensation screen signal SC in the left half L. Similarly, the high frequency components in the main screen signal SM of the right half R cannot be compensated only by the compensation screen signal SC in the right half.
The reason is that the compensation screen signal can be inserted such that the high frequency components of the main screen signal can be compensated if the compensation screen signal inserted into all the non-image screen portions 1 and 2 is used. If a compression process is accomplished, the high frequency compensation signal is unstable and this signal cannot be employed effectively.
There is a problem in the vertical wipe operation. As shown in FIG. 11c, the compensation screen signal in the upper screen U consists only of the horizontal high frequency components in the main screen of the upper screen U after the wipe operation is finished. After the wipe operation, the compensation screen signal in the lower screen L consists only of the vertical high frequency component in the main screen of the lower screen L. Thus, the resultant signal is completely different from the ideal signal.
These problems occur when the screen insertion position of the compensation screen signal has no relation to that of the main screen signal.
In order to resolve these problems, EDTV decoders could be provided at the input terminal of the video switcher apparatus, the number of the EDTV decoders should be the same as that of the inputs, the main screen signal and the compensation screen signal should be mixed to produce a single EDTV signal, and then the video signal switching process should be done. In other words, the high resolution wide screen would be restored once.
The above discussed construction needs many decoders at the input side of the video switcher apparatus and is very expensive. In the broadcasting station, since video switcher apparatuses are apt to be connected in cascade, the discussed construction is not practical.
The decoded EDTV signal is generally of the line sequence type because of the high quality image of the EDTV signal. Thus, the video switcher apparatus receiving the decoded signal should be the line sequence type. As a result, it is impossible to use the current type apparatus and the cost performance of the discussed apparatus is not acceptable.
What is desired is a video switcher apparatus that resolves the above discussed problems and can avoid an improper video switching operation without using a decoder or the like.